During the granulation of blast furnace slag, cooling water is sprayed onto a flow of liquid slag by means of a spraying head, while this slag flow is falling down from a slag channel into a granulating basin. The liquid slag is quenched in the process, such that it solidifies and forms granulates.
For granulating one ton of blast furnace slag, between 4 and 12 m3 of granulating water is required. Consequently, with large blast furnaces, the granulating water throughput in a spraying head is between 1000 m3/h and 4000 m3/h. For the granulate quality, apart from the granulating water quantity, among others also the impulse and shape of the water jets impacting on the liquid slag flow are decisive. These parameters are largely determined by the spraying head.
The slag throughput is not constant. In order to ensure an economical granulation and a constant granulate quality, the granulating water quantity is to be adapted to the slag throughput, the pressure remaining constant.
In order to achieve this aim, the DE 4032518 C1 proposes a spraying head with a field of hole type nozzles and a controllable slotted nozzle. The latter comprises two flap wings to be pivoted in opposite directions and forming a nozzle slot the height of which can be adjusted. A longitudinal edge of each flap wing has an undulating design and is rotatably fitted into a cylindrical bearing cavity which extends in the floor surface or the top surface, respectively, of a rectangular nozzle channel across the flow direction of the water. The other longitudinal edge of the flap wing forms the lower or upper lip, respectively, of the nozzle slot in this nozzle channel. By pivoting the two eccentrically mounted flap wings into opposite directions, the distance of the two lips can be changed, i.e. the height of the nozzle slot is reduced or increased. The two flap wings are connected outside the nozzle channel with a counter rotating gear which is driven by an electromotor.
The spraying head described in the DE 4032518 C1, however, has a lot of disadvantages. For example, the counter rotating gear and the electric drive have to be relatively strong as the water flow exerts considerable moments on the two flap wings. Furthermore, the wear resistance of the introduced slotted nozzle leaves something to be desired. In many granulating installations, the granulating water in fact carries large amounts of slag sand as it is conducted in a closed cycle. The abrasion force of such a water-sand mixture is well-known to the person skilled in the art. In the slotted nozzle of the DE 4032518 C1, in particular the two lips of the nozzle slot are subjected to an increased abrasion and wear out relatively quickly. This wear results in a relatively inexact characteristic of the control system of the slotted nozzle. Moreover, fine slag sand can penetrate into the bearing cavities of the flap wings which could possibly block the flap wings.